Air pressure switch

I have a 1993 357 Pete, I use the cruse control to govern rpms when running the pto for the wet kit. I have been getting the cruse cutting out. I am looking for the pressure switch activated by the parking brake, my truck will not run on cruse in neutral without the brakes set. When it kicks out the check engine light will come on, if I work the park brake the light will flicker. The cruse works okay rolling down the road, but when I stop and set the brakes it fails. One other thing I am wondering about, is it failing to sense the neutral position of the tranny. I still go back to working the of the park brake. Thanks in advance for any ideas. Pete